If you haven't already done shocks/struts, springs, anti-roll bars, polyurethane bushings, no wonder you are complaining. Next, if you REALLY want to improve high speed driving characteristics, I'd suggest underbody work. In particular, having aluminum or carbon fibre, or fibreglass, or sheet metal plates made to cover the entire bottom of your car, with a diffuser rear end. This will both decrease your cd, as well as increasing your downforce. This will provide a higher speed limit, if you are power limited, better highway mileage, better high speed stability, better handling at high speeds.